Metal pipelines intended for implantation in the ground require protection from corrosion and other degradative environmental forces. Consequently, the patent literature is replete with references to various types of coatings and tapes for protecting these pipelines.
One such protective system which has achieved great commercial success utilizes the combination of a rubber-based primer coating applied to the outer surface of the metal pipe and then overwrapped with an adhesive tape comprising a rubber-based adhesive carried on the inner surface of a backing material, most preferably an impact-resistant material resisting damage from falling rocks and the like.
The present invention is directed to providing an adhesive tape having improved resistance to shear and hence referred to hereinafter as a "high shear" adhesive tape.
As known in the art and well described, for example, in U.S. Pat. No. 4,472,231 issued to Robert F. Jenkins, anti-corrosion protective adhesive tapes ("pipewraps") that are applied to inground pipeline structures are often subjected to rather severe long-term shearing forces derived from the surrounding soil. The magnitude of these shearing forces depends upon several factors, including: (1) the type of soil; (2) the tectonic forces surrounding the implanted pipeline; (3) the diameter of the pipe; (4) the axial site emplacement; and (5) the range of thermal expansion of the pipe as well as its contents.
As is known, frictional forces acting between the pipewrap and the surrounding soil are the primary source of shear stress. These frictional forces are here defined as the product of the frictional coefficient between the outer surface of the pipewrap and the soil and the normal force acting around the pipe. Since the coefficient of friction depends upon both the nature of the outer surface of the protective pipewrap as well as the surrounding soil, it will be understood to vary in different usages.
Other factors having importance in these considerations are the weight of the soil above the pipe, as well as the weight of the pipe, including its contents. In addition, since the normal force will vary depending on the axial position around the pipe diameter, the frictional force and hence the shearing force will also be found to vary around the diameter of the pipe.
The result of long-term shear forces on the pipewrap system is referred to as "soil stress". Soil stress on anti-corrosion protective pipewraps generally results from the structural shear forces which cause the pipewrap to creep along the pipeline peripheral surface.
Creep is, in essence, a long-term visco-elastic, or "cold-flow" phenomenon, common to all polymeric substances. The amount of creep, however, will depend upon the physical properties of the pipewrap's adhesive coating. Since these physical properties (i.e. modulus) will be temperature dependent, temperature becomes a decisive element in determining the amount of creep. At low temperatures, the propensity of the pipewrap to creep will be substantially reduced, while at elevated temperatures the likelihood of creep will be significantly increased, other factors remaining the same.
The aforementioned Jenkins U.S. Pat. No. 4,472,231 is directed to providing a pipewrap system providing increased shear resistance, i.e. a high shear pipewrap or adhesive tape. The starting point of the Jenkins inventive concept is thought to reside in the statement in the patent system to the effect that when a rubber-based adhesive system is crosslinked, (1) the resistance to creep is increased; (2) the overall dimensional stability is improved; and (3) it is more resistant to heat distortion. These crosslinking effects are said to be generally intensified as the crosslink density is increased and can therefore be controlled by adjusting the number of crosslinks in the adhesive coating.
The Jenkins invention as described and claimed in the patent is a specified crosslinking system which comprises the combination of a particular primer coating applied to the outer surface of the metal pipe and a particular overlying rubber-based adhesive tape. Specifically, the primer coating comprises a blend of natural rubber, resins and a lead oxide crosslinking activator coated with organotitanate prior to incorporation in the primer coating; and the adhesive tape comprises a polyolefin backing material carrying a blend of virgin butyl rubber and reclaimed butyl rubber and which is initially partially crosslinked with p-quinone dioxime crosslinking agent, a tackifier, and a residual amount of unreacted p-quinone dioxime crosslinking agent.
In the claimed system, it is stated that when the pipe is placed in the ground, in situ crosslinking occurs at the primer-adhesive surface as well as throughout the primer layer and the adhesive layer in the presence of the elevated temperature of the pipeline and its contents.
However, as stated in U.S. Pat. No. 4,946,529 issued to Elwyn G. Huddleston (one of the instant joint applicants) and assigned to The Kendall Company, assignee of the Jenkins patent as well:
"It will be noted that the two-component system of Jenkins relies upon what the patentee describes in essence as a high speed additional crosslinking obtained by employing p-quinone dioxime as cross-linker and metal oxide, preferably lead dioxide, activator surface-treated with organo-titanate. The increased speed obtained thereby was thought to be critical to the solution of the task of the invention. PA1 "While the patented system was entirely satisfactory in small-scale manufacture of an anti-corrosion pipewrap system, it nevertheless suffered from certain deficiencies making it impractical in the larger scale commercial manufacture of the system. PA1 "Specifically, it has been found that the operating conditions taught in U.S. Pat. No. 4,472,231 do not provide a procedure which is processable in a Banbury mixer in commercial production of the adhesive. Repeated attempts to implement the teachings of the '231 patent on production equipment immediately resulted in lumpy adhesive."
Accordingly, it was specifically acknowledged in a commonly assigned patent application that the invention described in the Jenkins patent failed to provide a high shear tape for commercial production.
Since a high shear tape which was capable of commercial manufacture was still desired, subsequent research and development at The Kendall Company to solve the lumping problem in the Jenkins system then resulted in two inventions by the aforementioned Elwyn G. Huddleston.
The first filed of these two Huddleston inventions, which issued as U.S. Pat. No. 4,946,529, solved the problem by providing a system in which no crosslinking occurs in the Banbury. Instead, the initial crosslinking required to provide an adhesive tape is obtained by employing in the Banbury mix a commercially available pre-crosslinked butyl ("Kalar", trademark of Hardman, Inc.). In other words, the rubber components to be admixed with the tackifier and other ingredients in the Banbury to form the "premix" will consist of partially pre-crosslinked virgin butyl and a non-crosslinked virgin butyl rubber. Optionally, a portion of the virgin butyl may be, and preferably will be replaced by reclaimed butyl rubber, in which event the premix will contain a blend of partially crosslinked virgin sbutyl, non-crosslinked virgin butyl and reclaimed butyl rubber. The resulting premix will be effectively free of any crosslinking agent other than any inconsequential trace amounts of unreacted crosslinker that may be contained in the rubbers.
In any case, it is stated that the degree of partial crosslinking of the butyl rubber prior to mixing with the other components in the Banbury to form the premix dispersion may vary within a wide range to provide a coatable rubber-based adhesive formulation. It may, for example, be on the order of 35 to 75%.
It is next stated in the patent that, in general, the proportions of partially crosslinked rubber in the total rubber blend will vary inversely to the percentage of crosslinking, i.e., the greater the percentage of crosslinking within the above-noted 35-75% range, the lesser the amount of pre-crosslinked rubber is to be in the premix. While the determination of the precise amounts which may be desired for optimum results will be within the expected judgment of the skilled worker, it may be said that the amount of partially crosslinked rubber to be employed in the premix will be on the order of from about 8 to about 48%, depending upon the degree of crosslinking, the remainder being virgin butyl and reclaimed rubber.
By way of further illustration, it is then recited that when a 55% pre-crosslinked butyl is employed, it has been found that the amount of this pre-crosslinked butyl should be on the order of from about 13 to about 30% by weight of the total rubber blend. In other words, the rubber blend in the premix should comprise from about 13 to about 30% by weight of 55% pre-crosslinked butyl.
In a separate mixing step, the premix as described above is then admixed with an effective amount of a crosslinking agent necessary for the inground in situ crosslinking of the primer, primer-adhesive interface and the further crosslinking of the adhesive coating itself, in accordance with the teachings of the aforementioned Jenkins patent. Without the addition of crosslinking agent for inground in situ crosslinking, there is not enough crosslinked rubber present in the adhesive to obtain the desired shear resistance.
In the paragraph bridging Cols. 8 and 9, it is reported that the Huddleston system gave "comparable satisfactory protection, including creep resistance, to the Jenkins '231 system. Specifically, after 48 hours conditioning [above the ground] at 85.degree. C. [185.degree. F.] the shear rate will not exceed 10--.sup.8 meters/second. It is to be noted however that the force exerted is not defined in either the Huddleston or the Jenkins patent.
While the Huddleston '529 system does in fact achieve improved resistance to creep and has enjoyed substantial commercial success for many years now, and still does, it nevertheless suffers from certain deficiencies. The most significant of these deficiencies is that it requires in situ crosslinking on the pipe and the heat required for this in situ crosslinking may not in fact be available. Even on hot gas lines, many weeks or months may pass before the line is so heated.
Moreover, with the passing years, due to changes in the temperature and the flow of liquids and gases through inground pipelines there is now a need for still greater shear resistance than what existed at the time of the Huddleston invention that resulted in his '529 patent.
Referring back to Jenkins, it is stated in Col. 2 that adhesive resistance to flow or creep is improved by introducing crosslinks between the component rubber chains. It is also stated that the "crosslinking effects are generally intensified as the crosslink density is increased, and can therefore be controlled by adjusting the number of crosslinks in the adhesive coating.
With this in mind, attention is now invited back to the Huddleston patent.
In discussing the ranges of pre-crosslinked butyl that may be employed (Col. 6 of the patent), it is stated that "the degree of partial crosslinking of the butyl rubber . . . may vary within a wide range to prove [sic] a coatable rubber-based adhesive composition" (emphasis added). ["prove" is obviously a typographical error. It is thought clear that "provide" was intended.]
In other words, while not explicitly stated in the patent, it is implicitly clear that the constraints on the amounts of crosslinked butyl in the adhesive are due to the ability to coat the product. If the adhesive is too viscous due to excessive crosslinking, it will not be "coatable" to provide a tape.
Accordingly, a need existed to provide a pipewrap carrying a rubber-based adhesive layer which has more than 13-30% by weight of the total blend mixture crosslinked 55%, i.e. with a maximum of about 16.5% of its crosslinkable sites crosslinked, as taught in the '529 patent and still not be so viscous as not to be coatable on a tape backing.
Secondly, the in situ crosslinking required by the Jenkins system requires heat in the ground. Lord Chemical Co., supplier of p-quinone dioxime crosslinker states that the minimum activation temperature for this to occur is around 165.degree. F. However, many if not most pipelines never reach that temperature. Even assuming arguendo that they eventually did achieve that temperature, there would still be a time lag in the added crosslinking protection by the contemplated in situ crosslinking which is necessary to provide the desired shear resistance. This time lag before the pipeline is fully protected by the overlying pipewrap may be weeks or even months in the ground at a temperature between ground temperature and the 165.degree. F. minimum activation temperature. During this time lag, the previously mentioned sheer forces are working on the pipe coating.
While the manufacturing constraints in a commercial system are of course of lesser concern than the performance of the product, it will nevertheless be readily understood that anything that contributes to the cost of manufacture is always of concern and, consequently, very significant improvement for the manufacturer and supplier of the product will then lie in a more cost-efficient method of manufacture of an otherwise similar product in terms of performance by the end user.
In addition to the purchase and storage of the "conventional" or non-crosslinked butyl, a source of supply must be found for the pre-crosslinked butyl, the cost of which, incidentally, is very high; or, alternatively it must be manufactured in house for use in the adhesive premix. Secondly, additional warehouse storage is required for the pre-crosslinked butyl. Next, a separate manufacturing step is required downstream from the Banbury premix in order to add the crosslinker required for the in ground crosslinking. Finally, crosslinker and activator must be present at this downstream step. As will be appreciated with anyone familiar with plant manufacture, it would be most desirable for any or all of these criteria to be obviated.
The second of the two Huddleston inventions alluded to above was the subject matter of Ser. No. 843,943 filed Mar. 25, 1986 and now abandoned. However, although abandoned, the subject matter is disclosed in Col. 4 of U.S. Pat. No. 4,692,352 of Huddleston. As disclosed therein, the '943 application relates to an alternate approach to solving the problem with the Jenkins system wherein the partially crosslinked premix is provided by crosslinking in the Banbury with a phenolic resin crosslinking agent in lieu of the p-quinone dioxime of Jenkins. It is disclosed that "the premix will comprise a mixture of virgin butyl rubber and/or halogenated butyl rubber alone or in combination with reclaimed rubber, the virgin and/or halogenated butyl rubber being partially crosslinked by the phenolic resin crosslinking agent.
The last-mentioned patent application suffers from some of the same deficiencies previously mentioned. Specifically, shear resistance will not be provided until the inground temperature of at least 165.degree. F. required for in situ crosslinking is reached. As heretofore noted, this may not in fact occur. Even if it does, there will be a time lag in the protection to the pipe both above ground and after inground implantation until this temperature is reached.
The foregoing detailed discussion, which Applicants consider necessary for a full comprehension of the nature and objects of the present invention, constitutes all of the prior art known to Applicants relating to high shear rubber-based tapes at the time of the invention described and claimed in the aforementioned copending application, Ser. No. 08/859,189 (hereinafter "the parent case"), of which the present application is a continuation-in-part. It will of course be understood that further art not presently known to Applicants may in fact exist.